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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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COLUMBIA GLACIER                                     273
This push moraine was in part a feature produced by the 1892 advance, for there were dead trees in it overturned by the previous advance, and a photograph taken in 1899 shows the moraine crossing the heath much as in 1909, though with a very different relation to the ice front. In August, 1909, the ice was just extending beyond the 1892 maximum, as was shown by fresh clods of turf, stones, and pieces of wood that had rolled down the outer slope of the push moraine and by one or two blocks of ice that had tumbled down the outer side. We are not certain whether the moraine here had been moved forward at all or had been made higher in 1909.
By September, 1910, there was still more advance, but the glacier terminus was much thinner. This spectacular decrease in height of the ice terminus from 60 feet in July to less than 15 feet in September suggests either (a) tremendous vertical ablation (45 feet removed in 63 days, or nearly nine inches in a day), or else (b) the collapse of this portion of the glacier surface by flowing away of the ice beneath.
In this eastern part of this heath or meadow the push moraine was fronted by an area of turf ridges or billows, extending parallel to the ice front and one hundred and fifty or two hundred feet out from it. In 1909 there were synclines and anticlines of peaty soil two to six feet wide and three or four feet high, with grass and shrubs still growing upon them. Some of them had been made by the thrust of the ice during the 1892 advance, for they were photographed in 1899. At least two such peaty bolsters were, however, added by the 1909 advance at this point. These are not rolls that were invisible because submerged in the adjacent pond in 1899, for the water stood higher in 1909 than ten years before. The advance at this point extended, therefore, a trifle farther in August, 1909, than the 1892 maximum.
In July, 1910, the push moraine was quite similar to that in August, 1909, except that the whole morainic ridge had been pushed forward and it continued to advance during the summer of 1910. The sloping heath in front of it, however, was replaced by additional bolsters of peat. The depression between the ice and the push moraine was decreased somewhat in depth between 1909 and 1910. The push moraine rose 10 or 12 feet above the ice of the glacier terminus, and 22 feet above the lake to the south. It was made up of till with enormous striated bowlders, rounded gravel, and peat. It was covered with grass except in places where cracks interrupted and upon it were a few trees, most of them dead or dying, as a result of the advance of 1892. These trees rode forward on the back of the moraine from 1909 to 1910, maintaining their relative positions but slanting at different angles. As already stated, a number of peat rolls were added during this period of advance, the whole heath which sloped forward in front of the moraine in 1909 being destroyed before September, 1910. In some places two or three peat rolls were added between July and September, 1910. Some of these arches of peat were six feet high, mnm'ng parallel to the terminal moraine. In a few cases the pressure had resulted in closing up the folds to tightly-packed isoclinal structures, in others the arches had cracked across their axes, revealing the internal structure. The lake in front of this, portion of the ice front was of about the same size in 1909 and 1910, but of different shape and of slightly greater depth when the 1910 observations were made.
At the eastern end of this heathy meadow was a group of overturned and inclined trees, some dead and a few living. These merged into the eastern timber belt. Through this grove extended a push moraine made up of an intricate jumble of till, gravel,